The present invention relates to a magnetic resonance imaging method and apparatus employing a periodically switched readout gradient.
A magnetic resonance imaging method which is known as the echo planar method has been proposed in the Journal of Physics C: Solid State Physics, 10, L55, 1977. According to this method, a series of echo signals of excited magnetization are generated and measured by simultaneously applying two kinds of field gradients, i.e., a readout gradient which is switched alternately between gradients of opposite polarity and a continuous phase encoding gradient which is applied along a direction orthogonal to the readout gradient.
Another method known as Hutchison's method utilizes a pulsating phase encoding gradient, which performs rectangular signal scanning in Fourier space, instead of the continuous phase encoding gradient.
According to these methods, signal data for two-dimensional imaging is acquired after a one time excitation of magnetization. Therefore, the measurement time in these methods is much shorter than the measurement time in a magnetic resonance imaging method utilizing multi-time excitations. Various modifications have been proposed for these methods and all of the modifications can be considered to be ultra high-speed magnetic resonance imaging methods.
In general, it is very difficult to adjust the waveform of the readout gradient for obtaining appropriate images by the ultra high speed magnetic resonance imaging method. For example, an eddy current induced in a metal member located near a gradient coil or an offset in the output of a gradient coil driver causes variation in peak position of echo signals in respective phases of the readout gradient, as is described in U.S. Pat. No. 4,859,946 which corresponds to Japanese Patent Application Laid-open No. 86959/1989 and which discloses an imaging system having a peak detection mode in which a phase encoding gradient is not applied and an imaging mode. Peak positions of respective echo signals generated in the peak detection mode are stored in a memory. Sampled data of respective echo signals generated in the imaging mode are stored in another memory and read out for image reconstruction from respective positions of the memory which are corrected according to the stored peak position data. Another imaging system as described in U.S. Pat. No. 4,859,946 includes a correction unit for correcting the waveform of a gradient coil driver in the imaging mode on the basis of the stored peak position data. Both systems are intended to perform an automatic adjustment for obtaining appropriate images. However, it is difficult to perform precise adjustment corresponding to peak position deviations within one sampling interval in such systems.
An imaging system described in U.S. Pat. No. 5,055,789 which corresponds to Japanese Patent Application Laid-open No. 215440/1990 obtains a final image by synthesizing an MR image constructed from echo data acquired when the readout gradient is in positive polarity phases and another MR image constructed from echo data acquired when the readout gradient is in negative polarity phases. Position errors or intensity distribution errors in the both MR images due to inhomogeneity of the static magnetic field or frequency mismatch are respectively corrected before image synthesizing to obtain the final image free from artifacts.
According to an article in Medical & Biological Engineering & Computing, Vol 29, Supplement 1991, by the inventors of the present invention, the phase difference resulting from forward and reverse sampling under an inhomogeneous field in an ultra high-speed magnetic resonance imaging method is determined, and then used for matching in image reconstruction.